In general, a semiconductor device (e.g. an integrated circuit, also referred to as IC, chip, or microchip) may be processed in semiconductor technology on and/or in a substrate (also referred to as a wafer or a carrier). The substrate may include a plurality of semiconductor chips which are processed in corresponding regions of the substrate. For fabricating such semiconductor devices, certain electrical contacts are provided, e.g. using soldering.
Conventionally, lead based solder material is used in semiconductor technology to provide high temperature resistance and good mechanical properties of the achieved electrical contacts. However, lead is a toxic element which causes ecological damage and damage to health. Further, certain local provision of the law regulate (e.g. limit or forbid) the usage of lead containing solder material or plan to regulate its usage. Therefore, lead-free materials are desired to replace currently used lead based solder material in semiconductor technology.
Lead-free solder material complicates soldering processes, since its compatibility to conventional contact pads and semiconductor technology is smaller compared to lead based solder material. In most cases, lead-free solder material is limited in its temperature resistance and potentially melts during further processing the semiconductor device, e.g. during connecting the semiconductor device to a peripheral device, e.g. a circuit board. Further, lead-free solder material is limited in its capability to contact (e.g. fully wet) certain materials and tends to drip off from conventional contact pads and/or tends to form voids. This results in deficient electrical contacts causing potential malfunction of the corresponding semiconductor device. The front side contact pads of a semiconductor device require high precision soldering with narrow process tolerances, small contact areas, high electrical conductance, and high stability according to further process steps, which prevents the adaption of common lead-free solder techniques.
Conventionally, silver-epoxy adhesive is used to replace common lead based solder material in contacting processes. On the one hand, the silver-epoxy adhesive enables to use conventional equipment to dispense the silver-epoxy adhesive in analogy to solder material, which minimizes necessary changes in semiconductor processing. On the other hand, silver-epoxy adhesive is limited in contact reliability and its thermal resistance.
Alternatively, nickel (Ni) alloys are used to provide a solderable contact area, which in combination with tin (Sn) based solder materials (e.g. lead-free or also lead containing solder material) form NiSn intermetallic phases, which provide a good electrical and mechanical contact between Ni and the solder material. These nickel alloys may be used e.g. in the power device technology and/or in automotive technology. However, Ni requires an additional protection to avoid corrosion and an additional adhesion to the base metallization (e.g. Al(Si)Cu). For example, nickel oxide is hard to crack and prevents the formation of NiSn intermetallic phases. In the case of sputter deposition or in case of evaporation deposition, Ti is used as electrical contact and mechanical adhesion layer over AlCu or AlSiCu contact pads followed by a Ni or NiV layer as solderable compound material followed by Ag or Au as protection layer against nickel oxide formation.